Silver pastes such as resin silver pastes and calcined silver pastes are often used for forming wiring layers, electrodes and the like in electronic devices. Such silver pastes are thermally cured or calcined after applied or printed to thereby form conductive films that serve as wiring layers, electrodes and the like.
For example, a resin silver paste is composed of silver powder, a resin, a curing agent, a solvent and the like. The paste is printed on conductor circuit patterns or terminals, thermally cured at 100° C. to 200° C. to form conductive films, which are used to form wiring and electrodes. Alternatively, a calcined silver paste is composed of silver powder, glass, a solvent and the like. The paste is printed on conductor circuit patterns or terminals, thermally calcined at 600° C. to 800° C. to form conductive films, which are used to form wiring and electrodes. On the wiring and electrodes formed by these silver pastes, silver powder is disposed in lines to form electrically-connected current paths.
Silver powder used for silver pastes has a particle diameter of 0.1 μm to several μm, which varies depending on the thickness of wiring and electrodes to be formed. Alternatively, homogeneous distribution of silver powder in a paste enables formation of wiring and electrodes having a homogeneous thickness.
Although characteristics required for the silver powder for silver pastes vary depending on applications and use conditions, it is common and important for the powder to have a homogeneous particle diameter, little aggregation, and a high dispersibility in pastes. When the silver powder has a homogeneous particle diameter and high dispersibility in pastes, curing or calcination proceeds homogeneously to thereby permit formation of conductive films having low resistance and high strength. When the powder has non-homogeneous particle diameter and poor dispersibility, silver particles are not homogeneously present in a printed film. Thus, not only thicknesses of wiring and electrodes, but also curing and calcination become non-homogeneous, so that it is likely that conductive films have increased resistance or become brittle and fragile.
Furthermore, an important requirement for the silver powder for silver pastes is to be produced at a low cost. This is because silver powder, which is a main component of pastes, accounts for a large proportion of the price of the pastes. In order to reduce a manufacturing cost, it is important not only a low unit price of raw materials or materials to be used, but also low treatment-costs of waste fluid and exhaust gas.
In production of the silver powder described above which is used for silver pastes, a raw material commonly used as the silver source is silver nitrate. For example, Patent Document 1 discloses a method for obtaining homogeneous silver powder, wherein the method includes continuously mixing a solution containing a silver amine complex, which is provided by dissolution of silver nitrate in ammonia, and a reductant solution.
According to the production method shown in the Patent Document 1, it is possible to obtain granular silver powder which has an average particle diameter of 0.1 to 1 μm, is homogeneous, and is difficult to aggregate. However, silver nitrate emits poisonous nitrous acid gas in the course of dissolution in ammonia water and the like, so that a device for recovering the gas is required. Alternatively, a large amount of nitric nitrogen and ammonia nitrogen is included in the effluent, and thus a device for treating the nitrogen is also required. Additionally, silver nitrate is a hazardous substance as well as a deleterious substance, and thus it is necessary to handle carefully. In this way, when silver nitrate is used as a raw material of silver powder, a problem exists that influences and risks of silver nitrate on the environment are more severe than those of other silver compounds.
Accordingly, a method for producing silver powder is also suggested, wherein silver chloride is reduced without using silver nitrate as the raw material. When silver chloride is used, there exist advantages, such as lower disposal costs and lower environmental risks because no nitrous acid gas is emitted when silver chloride is dissolved in ammonia water. Additionally, silver chloride is neither a hazardous substance nor a deleterious substance, and has an advantage of being a silver compound relatively easy to handle although requiring light shielding. Silver chloride is also an intermediate of a silver purification process and has purity sufficient for electronics industry.
Patent Document 2 discloses a method for obtaining silver powder, wherein silver chloride is dissolved in ammonia water to provide a silver solution, to which a dispersant and a silver particle slurry are added, and then a reductant, hydrazine is added. However, the particle diameter of the silver powder obtained by this method was 0.2 to 3 μm and had a problem in the homogeneity.